A family of Microsoft word processing software products for creating web, email, and print documents.
Here's another possible cause:
In the document where the watermark is not showing, go to File > Options > Display and look at the first option, "Show white spaces between pages in Print Layout view". If that option is not checked, the document is displayed without headers or footers to make it easier to read from one page to the next. What isn't obvious is that it also hides anything -- including any watermark -- that is anchored in the header or footer.
If the space between pages appears as just a thin line, you can either check the option, or position the mouse cursor on that line between pages and double-click.